Optical communication device and system using optical power and signals

ABSTRACT

An optical telephone and communication system with interferometers at the transmitter and receiver can use optical time-delay difference and differential frequency shift modulation and multiplexing techniques. The power to the system is provided from either a remote optical source through the optical network or a local optical source. The optical network provides one or two optical connections to the sets. Part of the optical signal from the network is converted to electrical energy stored in a rechargeable battery. The optical sources used have a known coherence function, wavelength and coherence length Lc. The transmitter converts the information to phase variation in one arm of the transmitter interferometer and the receiver recovers this signal by using a matched interferometer. The path imbalance between the two arms of either interferometer is greater than Lc. Each set is assigned a particular time delay difference and/or differential frequency shift for addressing.

This application is the US national phase of international applicationPCT/GB02/02685 filed 17 Jun. 2002 which designated the U.S., the entirecontent of which is incorporated herein by reference.

The present invention relates to an optical communication device, anoptical communication system and/or a method for optical communication.

It is known to use interferometric modulation in optical transmission.For instance, it is known to provide optical signals over an opticalfibre by using an unbalanced interferometer at the transmitting end, amatched unbalanced interferometer at the receiving end and a lightsource of relatively short coherence length. An optical signalcontaining information can then be transmitted between the twointerferometers.

Examples of interferometric modulation are described by S. A. Al-ChalabiB. Culshaw and D. E. N Davies in “Partially Coherent Sources inInterferometric Sensors”; First International Conference on OpticalFibre Sensors, 26-28, Apr. 1983, pp 132-135, published by the Instituteof Electrical Engineers and the content thereof is hereby incorporatedby reference. A further example is disclosed in U.S. Pat. No. 5,606,446assigned to Optimux Systems Corporation

The terms “coherence function” and “coherence length” are both used inthis specification in relation to optical sources used ininterferometric modulation. It is known that to use interferometricmodulation, the optical paths in the interferometers should bemismatched by an optical path difference which is greater than thecoherence length of the optical source carrying the modulation.“Coherence function” and “coherence length” are both known terms and,for the purpose of providing background to the present invention,techniques for measuring them are described as follows with reference toFIGS. 4 and 1.

Referring to FIG. 4, the coherence function of an optical source 237 ismeasurable using a four port (two input ports and output ports)interferometer, such as a Mach-Zehnder, with two arms with an initialpath imbalance of zero. The source 237 is used as an input to theinterferometer and optical intensity is measured at one or more outputs270/272, 425 while the path length difference between the two arms ofthe interferometer is varied. A plot of measured optical intensityversus path imbalance, or time delay difference, can be used to give thecoherence function of the source.

The measure optical intensity includes a DC (direct current) componentand a coherence function component. It is known that the coherencefunction component undergoes a sign reversal in one arm of theinterferometer only, due to a phase change at a beam splitter. Hencewhere the optical intensity measured at a first output equals the DCcomponent plus the coherence function component, the optical intensitymeasured at a second output equals the DC component minus the coherencefunction component. This offers a simple way to extract the coherencefunction component and separate it from the DC component: photodetectorsare used to measure the optical intensity at two different output ports270/272, 425 of the interferometer and then the output currents of thetwo photodetectors are subtracted. As long as the splitting ratio of thebeam splitters used in the interferometer is 1:1, then the coherencefunction component can be extracted by simply subtracting the currentsfrom photodetectors at the two outputs thus removing the DC component.

Reference to this type of effect in beam splitters and interferometerscan be found, for example, in “Coherent Lightwave CommunicationSystems”, by ShiroRyu, published by Artech House in 1995, and in U.S.Pat. No. 4,860,279 entitled “Source modulated coherence multiplexedoptical signal transmission system” invented by: Coleman Jeffrey O andFalk R Aaron, with particular reference to FIG. 4.

Referring to FIG. 1, the coherence length of a source can be measuredfrom the same data. The measured photodetector outputs will showmultiple maxims (shown as a continuous line) as the path imbalance isvaried, this indicated the presence of the interference fringes. Theenvelope of the plot, shown as a dotted line, gives the coherencefunction of the source. At some particular value for the path imbalance,the measured photodetector outputs will stop showing furtherdistinguishable maxima and this particular value for the path imbalancecan be used as the basis for a workable measure of the coherence lengthL_(c) of the source. Different conventions have been used in the pastfor deciding L_(c). For instance, it may be measured as the pathimbalance at which the coherence function envelope has dropped by apredetermined percentage, for instance 80%.

In practice, a workable value for L_(c) may be affected by equipmentfactors such as noise produced by the bandwidth of the photodetectorused for intensity measurement. A useful definition of coherence lengthof a source for use in embodiments of the present invention might thusbe based on the maximum discrepancy in path differences between atransmitting and a receiving interferometer for which interferencefringes created by the transmitting interferometer using the source canstill be detected at the receiving interferometer. It can be expressedaccording to:(speed of light)×1/(spectral width of the power density of the source)

A method for plotting a coherence function for use in measuringcoherence length based on this useful definition follows. It might benoted that the coherence function (which is the same as theauto-correlation function) and optical power spectral density are aFourier Transform pair. This means that measuring one will give theother. The coherence function, and hence the coherence length, can bedefined as the speed of light divided by the optical spectra width.

A four port receiving interferometer with two arms is given the samepath length imbalance as a similar transmitting interferometer. Nointerferometric modulation is applied and the optical power spectraldensity at the output of the transmitting interferometer is keptconstant. The coherence function can then be obtained by introducing adiscrepancy in the path length imbalances of the two interferometers andplotting an output of the receiving interferometer against a range ofvalues for this discrepancy. In detail, the output of the receivinginterferometer which is plotted is obtained by subtracting the outputcurrents of photodetectors at the two output ports of the receivinginterferometer.

It should be noted that the method described above is for measuringcoherence length when equipment factors are taken into account. In orderto carry out the method, the path imbalance of the transmittinginterferometer must be larger at all times than the absolute coherencelength of the optical source which needs to be measured first toestablish its value, for instance using a spectrum analyser.

According to a first aspect of the present invention, there is providedan optical network for carrying communication signals to or fromtransmitting/receiving apparatus in optical form, the networkcomprising:

-   i) at least one optical power source for delivering optical power to    the transmitting/receiving apparatus for conversion to electrical    power at the transmitting/receiving apparatus; and-   ii) at least one optical signal carrier source for transmitting an    optical signal carrier to the transmitting/receiving apparatus for    interferometric modulation at the transmitting/receiving apparatus    and subsequent transmission over the network as an    information-carrying optical communication signal.

In general, the term “interferometric modulation” is used herein to meanmodulation applied at an interferometer to an optical carrier input tothe interferometer. It encompasses any detectable effect imposed on thecarrier by the interferometer, such as a phase or frequency effectapplied in an arm of the interferometer, whether or not aninterferometer is necessary for detection of the effect. It can alsoencompass any detectable amplitude or intensity effect applied in onearm of the transmitting interferometer or to the optical power at theinput or output of the interferometer. Such modulation might be used foraddressing and/or for leading information to a signal.

In practice, the optical power source and the optical signal carriersource may be the same piece of equipment, a portion of the opticalsignal carrier being used at the transmitting/receiving apparatus forconversion of delivered optical power to electrical power.

Some of the optical power delivered might be stored after conversion bymeans of a rechargeable battery and some of the optical power might beused directly on conversion to drive one or more aspects of thetransmitting/receiving apparatus.

It has been recognized in making the present invention that signallingof the general type described above, using interferometric modulation,is particularly appropriate in optical communications. This is very muchso in an optical network where the network is provided by fibre right toan end point, such as a data terminal or telephone, and in which poweris delivered to the end point by optical means. In such an environment,the power budget can be limited, potentially severely. However,interferometric modulation can be applied using very little, or evenzero, electrical power at the transmitting/receiving apparatus as longas an optical signal carrier is delivered to the transmitting/receivingapparatus to which the modulation can be applied. For example, movementproduced by acoustic means such as the voice can be used to modulate theoptical signal carrier along one arm of an interferometer and thusproduce a signal directly.

The question of electrical power supply at the receiving end arises forexample where public voice networks are concerned. Known telephonesystems use wires, mainly copper, to conduct electrical power totelephone apparatus. However, it is desirable in an optical network tosupply the required power over the optical connection/s used for voiceand/or data communication so that there is no need for any wires.Today's public telephone system provides electrical power to thetelephone apparatus from the telephone exchange or local switching boardto enable the apparatus to function without the need for an additionallocal power supply. This remote power provision makes it possible forthe apparats to be used in emergencies, such as fire or failure of theelectricity supply from the electricity grid. These similar requirementsare expected to be met by telephones and communications systems usingoptical communication systems deployed in public or private networks.

In recent years, the power levels of optical sees and amplifiers foroptical fibre systems have increased to the level where it is possibleto provide sufficient power over an optical connection to driveopto-electronic devices and to generate ringing sound (the telephonebell). These optical power levels may be limited however by the on-setof non-linear effects in the optical communication channel, or byinternational standards on safe optical power levels in opticalcommunication channels, and it is for this reason that the power budgetat transmitting/receiving equipment can be particularly constrained.

In embodiments of the present invention, it is feasible to construct avoice/data system which extracts all its power from the network. Aprimary attraction is that it is not necessary to employ an opticalsource in the transmitting/receiving equipment but to supply an opticalsignal carrier to it which can be modulated at thetransmitting/receiving equipment. This means a very significantreduction in the power required at the transmitting/receiving equipment.It is an option that an optical source is local to thetransmitting/receiving equipment but a remote optical source can beprovided as well or instead.

The main devices that will still need to be driven at thetransmitting/receiving equipment will depend on the application. Forinstance, in the telephony case this will include devices such asphotodetectors, biasing circuits, amplifiers, loudspeakers (ear piece),microphone, ringing bells, electro-optic modulator or piezoelectricdevices.

Embodiments of the present invention preferably use an interferometrictechnique for both signal modulation and addressing. Some form ofaddressing is usually important where different signals can be sent byany one of several transmitters (eg broadcasting where each transmitteris assigned an address) and/or received by any one of several receivers(eg telephony and/or data communication where each receiver is assignedan address). The addressing is used to identify either the transmitteror the receiver or both. To provide interferometric addressing, aninterferometer is used to apply a distinctive attribute to the opticalsignal transmitted by a optical path lengths of the two arms of aninterferometer are deliberately different by a predetermined amount,producing an “unbalanced interferometer”. As long as a transmittinginterferometer and a receiving interferometer are matched, then opticalsignals can be carried between them. Other distinctive attributes, inconjunction with time delay difference, such as frequency modulationproduced by a moving part in the interferometer might be used, or anamplitude modulation, produced for instance electrically at or inassociation with the interferometer.

The term “time delay difference” between the two arms of aninterferometer as used herein refers to the difference in time taken byoptical radiation to travel through each arm of the interferometer. The“time delay difference” equals the difference in optical lengths of thetwo arms of the interferometer divided by the speed of light. “Frequencyshift”, supporting frequency modulation, means the shift in the rate ofchange of phase of optical radiation which can be caused by a change inoptical path length in time. “Differential frequency shift” refers to adifference in frequency shift between optical radiation travelling inone arm of the interferometer with respect to optical radiationtravelling in the other arm of the interferometer.

Distinctive attributes of this type are not mutually exclusive and itwould be possible to increase the number of distinguishable addresses,and/or the information content, by using a combination of two or moredifferent types, such as both an unbalanced interferometer and afrequency or amplitude modulation.

Where an unbalanced interferometer is used for addressing, it is notnecessary that the transmitting and receiving interferometers are bothunbalanced so as to be perfectly matched. If two unbalancedinterferometers are perfectly matched, the difference in path lengthsfor each interferometer is the same as for the other. However, if twounbalanced interferometers are not perfectly matched, there arises adiscrepancy in the differences between their path lengths. Thisdiscrepancy should however be less than the coherence length of theoptical source used for signalling in order for a positive match to bemade and the fringes recovered at the receiver.

When the path inbalance of an interferometer is greater than thecoherence length of the source, no interference pattern is normallyobservable in the output of the transmitting interferometer (ignoringhere the more complex case where interference will be observed otherthan the pattern around the zero path length difference). However, whenthat output is fed to the receiving interferometer, the interferencepattern will be recovered as long as the discrepancy in the differencesin path lengths between the two if is less than the coherence length ofthe source L_(c). When the discrepancy is zero, i.e. the twointerferometers have identical path length differences, then theinterference pattern can be recovered to a maximum extent. As long asthe interference pattern can be recovered, any changes in the firstinterferometer can be tracked by the second interferometer which enablesan optical signal to be received.

Two or more in interferometers can be used to transmit multiplexedsignals over he same optical connection, and importantly can share thesame optical source, provided that the discrepancy in their path lengthdifferences is greater than the coherence length of the source Lo andpreferably significantly greater, such as three times eater when apartially coherent source is used This provides clean separation betweenthe signals intended for different receiving interferometers. Thus if afirst interferometer “n” has a path difference “L_(n)” and a secondinterferometer “m” has a path difference “L_(m)”, then theinterferometers can be resolved provided thatL _(n) −L _(m) >L _(c)where

-   L_(n): is the path length difference of the nth interferometer-   L_(m): is the path length difference of the mth interferometer-   L_(c): is the coherence length of the optical source

Thus there might be provided a multiplexing optical communication systemas an embodiment of the present invention, which system comprises:

-   i) an optical network for carrying multiplexed communications    signals; and-   ii) at least first and second optical modulators for use in    modulating one or more optical carriers to produce the    communications signals,    wherein each of said first and second optical modulators comprises    an interferometer having, in use, a difference in the optical paths    through it, and wherein there is a discrepancy between the    difference in the optical paths through the first optical modulator    and the difference in the optical paths through the second optical    modulator, said discrepancy being greater than the coherence length    of the one or more optical carriers.

In such a system, the communications signals produced by the respectiveoptical modulators can be distinguished at receiving apparatus and cantherefore be multiplexed in the optical network, along the same fibre ifnecessary.

The system may further comprise one or more optical sources forproviding the one or more optical carriers for the communicationssignals.

It is not necessary that the optical carriers have the same coherencelength, as long as the above condition regarding said discrepancy is notbroken.

It is again useful that the system uses interferometric addressing. Apair of interferometers can then be selected for making a communicationsconnection from one to the other in the manner of dialling a telephonenumber in voice communications. Information about the precise nature ofthe imbalance which must be matched by one unbalanced interferometer inorder to communicate via another, target unbalanced interferometer canfor instance be stored with respect to equipment connected to thenetwork, again in the same manner as conventional switching or routingin communications.

In embodiments of the present invention, there is more than one way inwhich a path length difference can be applied. It can for instance be astatic path length difference, measurable for example as a phasedifference in the optical radiation output from respective arms of aninterferometer, or it can be a dynamic change in path length. This canbe measured as a frequency difference in the optical radiation outputfrom respective arms of the interferometer: ie a Doppler shift.

Thus in embodiments of the present invention, there may be provided anoptical communication system, the system comprising:

-   i) an optical network for carrying communications signals; and-   ii) at least one optical modulator for use in modulating one or more    optical carriers to produce a communications signal, the modulator    comprising an interferometer having, in use, a difference in the    optical length of the optical paths through it    wherein said optical modulator is adapted to modulate by varying    said difference in optical path length.

It might be noted that if the path length difference is applieddynamically, it is possible but not essential that transmitting andreceiving interferometers in such a system also show a matchingimbalance in the sense of having one arm with a permanently differentpath length. If they are also unbalanced in this way, the number ofaddresses available can be greatly increased since there are then twovariables available for each address, the static path length differenceand the Doppler shift or differential frequency shift.

Either or both of the path length difference and the Doppler shift couldalso or instead be used to carry information rather than just providingaddressing. For instance, changes in path length are detectable and cantherefore themselves carry information while changes in the Dopplershift applied at the transmitter can also clearly be used to conveyinformation.

In practice, it is also possible to use amplitude modulation at atransmitter as either an addressing component or to carry information.This again extends the number of addresses that would be available,and/or the amount of information an optical channel can carry. Amplitudemodulation would not usually be applied as interferometric modulationbut by a separate amplitude control device or changes in drive to anoptical source.

Conveniently, each piece of transmitting/receiving equipment can act aseither a transmitter or a receiver, in the manner of a telephone orcomputing apparatus. Alternatively, for instance for use inbroadcasting, there may be provided at least one piece of equipmentdesigned for transmission or reception only.

Each piece of transmitting/receiving equipment can be identified on thenetwork by assigning its interferometer(s) a specific path lengthimbalance and/or a specific differential frequency shift between the twoarms of the relevant interferometer.

According to a second aspect of the present invention, there is providedoptical receiving apparatus for receiving optical signals over anetwork, which apparatus is provided with at least two interferometersfor receiving interferometric modulation, a first interferometer beingarranged for use in detecting a first type of modulation in an incomingsignal and a second interferometer being arranged for use in detecting asecond type of modulation in an incoming signal.

The first type of modulation might for instance comprise phase orfrequency modulation and the second type of modulation might forinstance comprise intensity modulation.

Preferably, the at least two interferometers are differently unbalanced,the discrepancy between the path length differences of theinterferometers being equal to, or approximately equal to, a quarter ofa central wavelength of an optical carrier carrying the incoming signal.This gives optimal detection of both types of modulation,phase/frequency and intensity, and can be described as theinterferometers being in “quadrature”.

“Central” in this context may mean average, or centrally placed in arange of wavelengths, or other similar qualification found appropriatein particular circumstances of use.

Conveniently, the optical receiving apparatus may further comprise apath length control component for producing a path length change inrelation to both interferometers at the same time. This is particularlyefficient where there is a first interferometer which is used to detectphase or frequency modulation and a second interferometer, in quadraturetherewith, which is used to detect intensity modulation. The firstinterferometer might be used to track the phase or frequency modulationand the tracking result can be used in a feedback loop to the pathlength control component to both interferometers at the same time, sothat both interferometers can be adjusted to follow said phase orfrequency modulation.

It will be understood that references to path length in this context arereferences to optical path length. Therefore the path length controlcomponent could be anything capable of changing optical path length,such as a piece of electro-optic material which changes refractive indexin response to changes in an electric field, or a piezo-electricmaterial which changes the physical length of the path. However, aconvenient arrangement is one in which the at least two interferometersshare at least one reflector as the path length control component, thearrangement being such that movement of the shared reflector results insaid path length change in relation to both interferometers.

In an embodiment, the at least two interferometers might be provided byone shared pair of reflectors, at least one of which is aretroreflector, the apparatus further comprising at least one input beamsplitter, the arrangement being such as to provide multiple opticalpaths in relation to the reflectors which can be brought together asoutputs to constitute said at least two interferometers.

The invention also encompasses optical transmitting apparatus for usewith optical receiving apparatus according to the second aspect, thetransmitting apparatus comprising means for applying intensitymodulation to an optical signal carrier.

It has further been realized in making the present invention that it hasadvantages in a wavelength division multiplexed communication system inthat it can provide an extra channel, outside the wavelength divisionmultiplexed channels.

According to a third aspect of the present invention, there is provideda receiver for use in receiving wavelength division multiplexed signalsin optical communications, said receiver comprising a filter forfiltering out one or more wavelength ranges carrying wavelength divisionmultiplexed signals from an incoming optical carrier, wherein thereceiver further comprises an interferometer for use in detectinginterferometric modulation in a portion of the incoming optical carrierremaining after said filtering out.

The invention in this third aspect takes advantage of the use of aninterferometer to extract a signal from an optical carrier which hasalready had wavelength division multiplexing channels filtered out. Thatis, it has been recognized that there can be sufficient opticalbandwidth left in such an optical carrier to carry a detectable signal.

In relation to embodiments of the invention which comprise systems, theinvention may also or instead be embodied as a transmitter and/or areceiver for use in such a system. Further, in another aspect, anembodiment of the invention might be provided by a method ofcommunication within such a system, or by a method of communicationusing such a transmitter and/or receiver.

Power delivery by optical means is known. An example in an optical bussystem is disclosed in European patent application EP 1026839 in thename Phoenix Contact GmbH & Co. In most circumstances, the opticalsources used to provide the power and optical signal to the network inan embodiment of the present invention should produce as much power aspossible and have well defined wavelength and coherence functions. Thesources used for communication and providing power should preferablyhave a well defined coherence function with a coherence length L_(c)which is less than a specified value. To ensure that the coherencefunction of the source is well behaved and independent of thereflections from the optical network, optical isolators and polarisationcontrollers might be required to reduce the effects of such reflections.These types of sources can be combined with optical sources of narrowlinewidth and specified wavelength to provide power to the opticalcommunication set. The latter sources are used only to provide power ata specific wavelength and are filtered by the transmitter and/orreceiver optical system to be converted to electrical power.

On or more embodiments of the invention will now be described, by way ofexample only, with reference to the accompanying drawing in which:

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a coherence function of a type relevant to an opticalcarrier;

FIG. 2 shows the general optical telephone and communication systemtogether with an optical network;

FIG. 3 illustrates the transmitter opto-electronic system withinterferometer, photocells, a transducer for converting the voicepressure to optical time delay, phase and/or frequency shift modulation,and data transducer converting the electrical signal representing thedata to optical time delay, phase and/or frequency shift modulation;

FIG. 4 illustrates a transmitter module with a local optical source,interferometer and phase, frequency or amplitude modulation for datacommunications system;

FIG. 5 illustrates the receiver for the optical telephone and datacommunication system for recovering voice and data information;

FIG. 6 illustrates an optical network built using passive opticalcomponents; such as optical fibres and optical couplers, and activeoptical components such as optical amplifiers;

FIG. 7 illustrates an embodiment of the invention using both phase andintensity modulation; and

FIG. 8 illustrates an embodiment of the invention used in combinationwith wavelength division multiplexing techniques.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF INVENTION

Network

Referring to FIG. 2, the optical telephone and communication systemcomprises one or more optical telephones and other communication sets210, 220, 230 and 240 together with one or more optical sources 250, 255and an optical network 260. The network can be built from passiveoptical components; such optical fibres and couplers, and active opticalcomponents such as optical power splitters, amplifiers and switches. Theoptical sources and communication sets are connected to the opticalnetwork through various types of optical connection 270, 272, 274 and276. For example, a first connection type 270 is one connection fortwo-way communication, a second connection type 272 connects thetransmitter module in a two way two connection communication, a thirdconnection type 274 connects the receiver module in a two way twoconnection communication, and 276 connects an optical source 250, 255 tothe network.

In general, the network should provide an optical path between two ormore optical telephones and communications modules with sufficientoptical bandwidth to enable the use of optical sources with a coherencelength less than Lc.

The communications sets 210, 220, 230 and 240 can be one of two generaltypes. The first type, such as 210, 220 and 240, does not have anoptical source and derives most if not all its power from the opticalsources 252 and 255 and other optical sources connected to the network.The second type of optical telephone and communications sets, such as230, contains an optical source, such as 237, and can derive some of itspower from a local electrical power source. All optical sources musthave a well specified optical power spectral density, or a coherencefunction, and those that are used for communication rather than justpower have a coherence length less than a specified value Lc whichshould be as short as possible.

The optical sources 250, 255 should be powerful enough to provide enoughoptical energy to operate the telephone and communication sets 210, 220and 240 of the first type. The main factors that limit the power levelsof these sources are safety and performance quality of the voice andcommunication channels. The optical power received by the opticaltelephone and communications and converted to photoelectric currentshould be sufficient, either directly or through a re-chargeablebattery, to drive the electronic, electromechanical andmagnetomechanical systems and devices to effect voice and/or datacommunication system.

Each optical telephone and communication set consists of a transmittermodule 211, 221T, 231, 241 and a receiver module 214, 224, 234 and 244.The transmitter module consists of an optical system part 213, 223, 233,243 with an interferometer of a specified path length imbalance greaterthan Lc, and an opto-electronic part 212, 222, 232 and 242 withelectronic components and transducers to convert an acoustic signal anddata to modulation of the phase, amplitude and/or frequency shift of theoptical signal in one or both arms of the interferometer in thetransmitter. The transmitter module can use an optical signal generatedby a remote or a local optical source, in either case with coherencelength less than Lc, for communication.

An embodiment of the optical network which provides two connections fortwo way communication is shown in FIG. 6 which consists of opticalfibres 600, optical couplers or power splitters 605, and opticalamplifiers which may be either unidirectional or bi-directional 610. Theconnection to the receiver module 500N of the Nth optical telephone andcommunication, for example the receiver for set 1 is 500A, and theconnection to the transmitter of the same Nth set, for example thetransmitter of set 1 is 300A. Similarly the connection to the receivermodule of the optical telephone and communication set 2 is 500B and theconnection to the transmitter of the same set is 300B.

Transmitter Module 211

Referring to FIG. 3, a transmitter module 211 which uses a remoteoptical source as a power source and signal carrier derives all itspower from the optical network through its optical connections 270 or272. The transmitter module 211 comprises a Michelson interferometer 300where the optical power splitter 330 splits the optical carrier comingfrom the network through connections 270 or 272 to two arms or paths 310and 335. The optical signal in each arm 310, 335 can be travelling ineither free space or guided media such as optical fibre or integratedoptical device. The optical path imbalance of the two arms 310 and 335is greater than the coherence length Lc of the relevant source andmatches the optical path imbalance of the two arms of the receiverinterferometer with which the communication is to be established.

The incoming optical carrier is modulated by introducing one or more ofa phase shift, a frequency shift and a time delay to one or both arms ofthe interferometer 300. This can be done using for example a voicetransducer 345 and/or a data transducer 315.

In more detail, a first reflector 305 in the Michelson interferometer isstatic, while a second reflector 340, which can be a reflectivediaphragm, is coupled to a voice transducer 345 which can convert anacoustical signal and pressure to phase and /or frequency modulation ofan optical signal in one arm 335 of the interferometer 300. The datasignal meanwhile can be imposed on the phase and/or frequency of theoptical carrier in one of the arms 310, 335 by the data transducer 315.

The voice diaphragm 340 can be displaced either directly by an acousticsignal, or after amplification of an electrical signal representing theacoustic signal, and acts as the reflecting mirror of one arm of theinterferometer. The motion of the diaphragm due to the acoustic signalwill cause phase modulation of the optical carrier in that arm. Thisphase modulation can also be referred to as a Doppler shift in theoptical frequency due to the motion of the reflector 340. The voicetransducer can have many other implementations including those usingguided optics or bulk optics where phase or frequency modulation isachieved using electro-optic, magneto-optic, piezoelectric transducers.For example, the mirror 340 could be provided as a piezoelectrictransducer which is driven by the telephone's microphone. However, allimplementations perform the same function of converting sound pressureto optical phase and/or frequency modulation. In general, the voicediaphragm needs to produce movement sufficient to be detected, that is,sufficient to cause a phase/frequency shift that can be detectedinterferometrically. This can be as low as a nanometer optical pathlength change. The diaphragm should also have sufficient sensitivity anddynamic range (say 40-50 db) to insure that the signal is not distorted.

The data transducer 315 comprises a phase or frequency modulator in onearm of the transmitter interferometer. As well as introducing data, thisphase/frequency modulator can also be used for addressing ormultiplexing purposes where a specified phase/frequency shift of theoptical signal in one arm of the interferometer with respect to theother arm can be used to assign each transmitter a specific code or apart of the available spectrum of differentially frequency shiftedsignals. Thus the data transducer 315 can be used for assignment of anaddress to a transmitting/receiving assembly by modulating/demodulatingthe phase/frequency at a distinctive rate—which is usually higher thanthat of the voice diaphragm 340.

In this general scheme, a difference in the time delays of thetransmitter and the receiver interferometers needs to be less than thecoherence length of the optical source, and each receiver with thisparticular time delay difference can be assigned a particular part ofthe spectrum of the differential frequency shifted signal. The phasemodulator when used for addressing should produce an effective opticalpath change greater than the central wavelength of the optical source,while phase modulation which conveys information/data should produce aneffective optical path change which is determined by the required depthof modulation which enables satisfactory demodulation. The datatransducer 315 can have many implementations including for exampleguided lithium niobate devices, electro-optic crystals,microelectromechanical systems (MEMS) and vibrating mirrors. However,all implementations perform the same function of converting informationto phase/frequency and/or amplitude modulation.

Referring to both FIGS. 3 and 4, as an alternative to the voice and datatransducers described above, an amplitude or intensity modulator 320,420 can be used in an arm or at the output of the transmittinginterferometer to apply data or voice signals delivered over aconnection 325 from the signal processor 365. Examples are anelectro-absorption device and a semiconductor optical amplifier whosegain can be controlled electronically. Another example is anacousto-optic modulators but these are usually bulky and require a lotof power to drive them.

Thus phase, frequency and amplitude/intensity modulation can be usedsimultaneously to transmit information.

The reflected signals from the two arms of the interferometer 300,having been through voice and data transducers 345, 315, are thencombined and re-split by the optical power splitter 330 to provide amodulated output signal to the network and a power input to thephotocells 350 of the transmitter module 211. That is, the powersplitter 330 outputs a first part of the combined signals to the opticalnetwork 260 through the optical connection 270 or 272 and a second partto the photocells 350 through an optical connection 360. The photocells350 convert incident optical power to electrical power which can bepartly or completely stored in a re-chargeable battery 355 which canthen provide electric power to drive the systems and circuits of anoptical telephone and communications set as required.

It is an option to use an optical source of a specified wavelength toprovide power only. In this case, an optical filter can be inserted atthe input to the interferometer 300 and the filtered optical power canbe taken directly to the photocells 350.

Hence the transmitter module 211 comprises:

-   -   an interferometer 300    -   an optical power splitter 330 to divide optical power received        from an optical source into a first part fed to photocells 350        and a second part fed to the network as a modulated signal    -   one or more photocells 350 for converting optical power to        electrical power    -   optionally an optical filter so that an optical power source of        a specific wavelength can be used to provide power only    -   a rechargeable battery 355 for storing the electrical power        generated by the photocells.    -   a voice transducer 345 to convert an acoustic signal generated        by the user to a phase or frequency shift modulation of the        optical carrier in one arm of the transmitter interferometer.    -   a data transducer 315 to convert information in an electric        signal to a modulation of the phase and/or frequency of the        optical carrier in one arm of the transmitter interferometer    -   a signal processor to process voice and data signals and to        generate suitable electrical signal to drive the voice and data        transducers 345, 315

Although the signal processor is not shown in FIG. 3, or discussed indetail above, in practice the signal processor 565 will usually beshared with the receiver module and is shown in, and discussed belowwith reference to, FIG. 5. In the transmitter, it performs any necessaryprocessing of signals prior to modulation and controls conversion ofoptical to electrical power for driving local components. Additionally,where the optical connection is via a single optical connection usedboth for transmission and reception, the transmitter might also have anoptical filter to separate different parts of the optical spectrum whereone part can be used for transmitting and the other part can be used forreceiving.

As mentioned above, the transmitter module 211 might use either a remoteoptical source or a local optical source. FIG. 4 shows the use of alocal optical source 237 for communication, which could in practice beused in conjunction with a remote source. Whether local or remote, theoptical source should preferably have a coherence length less than Lcand a well-defined coherence function. Further, the output power of thetransmitter module 211 should be sufficient to give a good signal tonoise ratio at the input of a receiver for adequate voice quality anddata communication whether digital or analogue.

In the arrangement of FIG. 4, the output of the optical source 237 isfed to a Mach-Zehnder interferometer 410 having a path imbalance betweenthe two arms 405, 400 which is greater than the coherence length Lc ofthe source and equals the path imbalance (or time-delay difference) ofthe two arms of the receiver interferometer with which communication isto be established. Data information can again be imposed on the phaseand/or frequency of an optical carrier in one or both arms by a datatransducer/modulator 415.

Receiver Module

Referring to FIG. 5, each optical telephone/communication set 210, 220,230, 240 will usually also incorporate a receiver module. FIG. 5 showsan embodiment of a receiver module 214, 224, 234, 244 together withphotocells 350 and a rechargeable battery 355.

The receiver module 214 comprises an interferometer 500 whose pathimbalance should match, to within less than the coherence length of thesource, the path imbalance of a transmitter interferometer sending voiceand/or data The preferred type of interferometers in the receiver arethose with two outputs such as an off-set Michelson or a Mach-Zehnder,although a resonator type such as Fabry-Perot could also possibly beused. The interferometer 500 may also have other optical components suchas polarisation controllers or optical dispersion compensators tomaximise the optical signal and match the spatial distribution ofradiation travelling in each arm of the interferometer.

A correctly addressed optical signal, received at a receiver module 214from the optical network through a connection 270, 272, will consist ofat least two components with a time-delay difference equal to the timedelay difference between the two arms 505, 510 of the receiverinterferometer 500. A voice channel will occupy the frequency spectrumwhich was allocated to it by the relevant transmitter while a datachannel will occupy other frequencies of the demodulated signal.

The optical outputs 520, 525 of the interferometer 500 are guided orfocused onto photodetectors 530, 535 which can be connected in series.The two photodetectors are connected in such a way as to generate adifference current proportional to the difference in intensities of theoutputs of the interferometer 500. The difference current is fed througha connection 540 to a signal processor 565.

The signal processor 565 receives both the difference current (I₁-I₂)and a signal directly proportional to the currents of the photodetectors530, 535 to extract voice and data information. That is, the twodetectors 530, 535 can also be connected to a circuit whose outputrepresents the total optical power from the two arms of theinterferometer. Such a circuit might comprise for example resistors 560,555 where the voltage across the resistors 560, 555 is proportional tothe sum of the photodetectors' currents. The voltages across theresistors 560, 555 are fed through high impedance connections 550 and545 to the signal processor 565.

The signal processor 565 processes the electrical signals to extract anddemodulate the transmitted voice and data information and to separatethe channels that are assigned different addresses such as differentdifferential frequency shifts or amplitude modulation frequencies.

There are several ways to process the signal from the photodetectors. Anoptimum way is to find a match between the time delay and differentialfrequency shifts of the transmitter interferometer and the receiverinterferometer to yield a maximum difference between the photocurrentsof the two photodetectors. In a preferred method, this can be done byrequiring the signal processor 565 to generate a first signal selectedfrom a set of signals agreed with the transmitting end and fed to afrequency and/or phase modulator 575 in one arm of the interferometer500, through the connection 580, and to generate a second signal to areflector 515 in the same arm of the interferometer 500, through afeedback system 570 and connection 585 of the receiver interferometer500. The first modulation signal is used to identify the addressingsignal and the second signal is used to recognise the phase modulationimposed by the transmitter.

For example, a sawtooth modulation of the optical frequency in thetransmitting interferometer can be used for addressing and theinformation/data can be conveyed by modulating the phase differencebetween the two arms of the transmitting interferometer where thesawtooth period is smaller than the information/data period. To identifythis signal, the receiving interferometer will apply a sawtooth signalto one phase/frequency modulator in one arm of the receivinginterferometer and another signal to the other phase/frequencymodulator. When the signal processor generates a sawtooth signal of thesame period and frequency shift as the transmitter, the data signal canbe demodulated by selecting the phase signal applied to the second phasemodulator in the receiver interferometer that matches the transmittedphase. Using this signal processing technique, the receiver can identifythe address of the transmitting interferometer and recover thephase/frequency information imposed at the transmitter interferometer.(The exact feedback signals and circuit will depend on the signalprocessing technique used to extract the voice and data information aswell as the time delay, phase and differential frequency modulators andcompensators.) These generated feedback signals can then be used todemodulate and recover the information from the phase and/or frequencydifference between the two components of the input to the receiverinterferometer with a time delay-difference equals the time-delaydifference between the two arms of the transmitter interferometer.

A less preferred method is to match the path imbalances of the receiverinterferometer to the transmitter interferometer and then pass thedifference current between the two photodetectors 530, 535 through abank of electrical filters to recover the data information. The outputof each electrical filter will correspond to a particular transmitterand the information/data can then be recovered by the signal processor.In the case of frequency shift keying scheme, where the information/datais conveyed by a change in the frequency agreed with the transmitter,the signal processor can then monitor the frequency from each electricalfilter and compare it using a frequency discriminator, or electricallycoherent detection, to decide which frequency was transmitted. Similarsignal processor can be used where the information/data is conveyed bychanges to the phase, and in this case a phase locked loop or a standardelectronic phase detection can be used to decide whether a one or a zerois transmitted in the case of a digital communication system. (This lesspreferred method may also might require the signal processor to generatesignals to drive feedback circuits for modulating the phase and/ordifference frequency shift of the optical signals in either arm of thereceiver interferometer.)

The optical system described in this invention can be used to enablevoice and data communication using analogue or digital modulationschemes. The system can use standard signalling protocols to establishand terminate a telephone call. This will require that the signalprocessor 565 can also ensure production of the electrical current todrive a standard telephone ringer 590, the voice signal to the earpieceor speaker 592 of the optical telephone and the recovered data to thedata channel 594 of the optical communication set The waveform to drivethe ringer will determine the ringing sound and the ringer can be of apiezoelectric or an electromechnical type requiring very small drivecurrent preferably less than a milli-ampere. The signal processor isalso used for demodulation and amplification of the voice signal andproducing the required electronic signal levels to drive the earpiece inthe telephone.

Thus a receiver module 214 as shown in FIG. 5 comprises:

-   -   an interferometer with path difference greater than the        coherence length Lc of the optical sources.    -   photodetectors for collecting the two outputs of the        interferometer    -   a signal processor    -   optionally a feedback circuit from the output of the signal        processor to drive phase and/or frequency modulators

The receiver might also have an optical filter to separate M differentparts of the optical spectrum for use by different channels. Forinstance, one part of the optical spectrum might be used fortransmitting and another part used for receiving, or an interferometricmodulation system according to an embodiment of the present inventionmight be combined with a more conventional system using wavelengthdivision multiplexing techniques.

For amplitude demodulation schemes, the receiver might also have twointerferometers in quadrature as explained in a later part of thisspecific description.

Addressing

As mentioned above, assignment of addresses to identify the opticaltelephone and communication sets in this invention can be based onassigning a particular time delay difference (greater than theequivalent coherence length of the optical source) and differentialfrequency shift for the optical interferometers in the receiver andtransmitter modules. The assignment of a particular time-delay and/ordifferential frequency shift to the optical interferometers depends onthe use of the communications system. The following communicationschemes are covered by this invention: one-to-one, one-to-many orbroadcasting, many-to-one, and many-to-many.

In the case of one-to-one communication, the receiver interferometer ofeach set is assigned a specific time-delay difference and a specificdifferential frequency shift The path imbalance of the transmitterinterferometer is adjusted to match the path imbalance of the receiverinterferometer within the coherence length of the optical source withwhich the voice and/or communication channels are to be established. Thevoice and data channels for that specific time delay difference can beallocated to one or more specified differential frequency shifts. In adata communication system, several sets can be assigned the same timedelay difference but they then have to be assigned differentdifferential frequency shifts to facilitate one-to-one communication

In the case of the one-to-many communication system, the transmitterinterferometer of one set is assigned a unique time delay difference anddifferential frequency shift. The time delay difference and differentialfrequency shift of the receivers' interferometers of the other sets mustbe adjusted to match that of the transmitting interferometer toestablish a one-to-many or broadcasting communication.

In the case of many-to-one communication, the receiver interferometer isassigned a specific time delay difference and one or more differentialfrequency shifts. The transmitting sets have to adjust their transmitterinterferometer to match this time delay and differential frequencyshift.

In the case of many-to-many communication within a group of transmittersand receivers, the receiver interferometer and the transmitterinterferometer of all communication sets in the group are set to thesame time delay difference and differential frequency shift. The timedelay difference of all interferometers in the receivers in the groupare set to match the time delay difference assigned to all transmittinginterferometers. The transmitting interferometers can then transmit atthe same differential frequency shift. Alternatively, each transmitteris assigned a specific differential frequency shift to avoidinterference, however, the receiver should be able to demodulate thedifferential frequency shifts of all transmitters.

Receiver Module With Double Interferometer

Referring to FIG. 7, the transmission system described above can be usedto convey information from a transmitter T_(x) to a receiver R_(x) bymodulating the intensity of an optical signal either by directlymodulating an optical source or by using an intensity modulator 730outside or inside the interferometer. This method can be used inaddition to transmitting information by modulating the path difference,and thus relative phase or frequency difference of the transmitterinterferometer. The transmitter interferometer can be the same as thatdescribed above, where the optical signal is supplied from the network.It has a characteristic path difference for addressing purposes but nowalso carries the intensity modulation as well as any phase or frequencymodulation.

To demodulate the intensity and the phase/frequency signals separately,the receiver has a pair of interferometers. A first receivinginterferometer of the pair is used to track the phase or frequencymodulation and the second receiving interferometer of the pair is usedto detect intensity modulation.

As shown in FIG. 7, only one pair of retroreflectors 705 and the beamsplitters 710 is used to provide both interferometers. This is done byusing a beam splitter 715 at the input so that some incoming radiationpasses straight to the first retroreflector 710 while the rest of theincoming radiation is diverted to a reflector 720 and reaches the firstretroreflector 710 at a different point. This produces four differentpaths through the retroreflectors 605, 710 as shown, two for eachinterferometer. The two paths for a first interferometer are representedby the solid lines received at the detectors D₁ and D₄ and the two pathsfor a second interferometer are represented by the solid lines receivedat the detectors D₂ and D₃. There is a path difference within eachinterferometer, because the radiation takes different paths through theretroreflectors, and an additional phase shift component 700 ispositioned in one of the paths of one of the interferometers so that thetwo interferometers show different respective path differences. Thediscrepancy in path differences provided by the phase shift component700 is a quarter of the central wavelength of the optical sourceproviding the incoming signal, putting the two interferometers inquadrature, and the reason for this is that:

-   -   the intensity modulation is best detected when the path        imbalance of a receiving interferometer is exactly matched to        the path imbalance of the transmitting interferometer    -   in contrast, phase/frequency modulation is best detected using a        discrepancy in the path imbalances which produces a phase delay        equivalent to a quarter wavelength of the carrier because        carrier intensity variation is then zero as detected at the        receiving interferometer.

As discussed above, FIG. 1 shows the effect of changing the discrepancyin path length differences between a receiving interferometer and atransmitting interferometer on the difference in photodetector outputsfor the receiving interferometer. (In FIG. 1, the discrepancy in pathlength is shown as the equivalent time delay, along the “x” axis.)Within the envelope shown as a dotted line (the coherence function 110for the optical source), the discrepancy in path lengths is varied froma negative value to a positive value, both of which are equivalent tojust over three times the central wavelength of the source. It can beseen that maximum output intensity is detected at the receivinginterferometer when the interferometers are exactly matched, seen as thepeak detected intensity 100. However, at a path imbalance of one quarterthe wavelength of the source, shown as a “a quadrature point” 105, zerooutput intensity is detected and this is the best path imbalance atwhich to detect phase/frequency demodulation.

Hence it is optimal to use a receiver which has two interferometers withdifferent path imbalances which are in quadrature, i.e. the discrepancyin their path length differences is a quarter of the central wavelengthof the optical source providing the signal carrier.

In the arrangement of FIG. 6, the modulation can be detected by usingfour photodetectors, D₁ through D₄. Each photodetector measures theoptical intensity at the output from one of the paths through aninterferometer. As shown, photodetectors D₁ and D₄ monitor the outputsof one interferometer (the “D₁/D₄ interferometer”) and photodetectors D₂and D₃ monitor the outputs of the other interferometer (the “D₂/D₃interferometer”). The D₁/D₄ interferometer is matched to different by aquarter wavelength of the source. Hence the two receivinginterferometers are in quadrature.

It might be noted that, in relation to this arrangement, the curve shownin FIG. 7 would be produced by plotting the difference of the outputs ofeither D₁ and D₄ or D₂ and D₃.

The phase or frequency modulation is recovered by monitoring the outputsof the detectors D₂ and D₃ of the D₂/D₃ interferometer. As soon as thesum of those outputs starts to rise, an error signal is generated. Theerror signal is used in a feedback loop which adjusts the pathdifference for both interferometers to return the sum of the outputs ofthe detectors D₂ and D₃ to zero. It does this for instance by moving oneof the retroreflectors. The error signal itself provides a detectionmechanism for the phase or frequency modulation.

The intensity modulation meanwhile can be simply recovered from thedifference between the outputs of the detectors D₁ and D₄ of the D₁/D₄interferometer. The two outputs respectively give a DC component plusmodulation intensity component but there is a phase inversion withrespect to the detected modulation intensity which has occurred due tobeam splitting which affects one arm only of the D₁/D₄ interferometer.Hence when the difference between the two outputs is used, the DCcomponent is cancelled but the modulation intensity component isdoubled.

This arrangement has the advantage that it rejects signals that areintensity modulated which are not generated by the transmittinginterferometer because the phase modulation of the transmittinginterferometer is being exactly tracked. To achieve this rejectioncharacteristic, the power splitting ratio of 1:1 between the two arms ofeach interferometer, including the one at the transmitter and both atthe receiver, should preferably be used. The power splitter at thereceiver input (marked “Input Splitter” in FIG. 6) does not however needto be 1:1 and it might be found preferable to adjust this powersplitting ratio so as to yield the best phase/frequency and intensitydemodulation available.

In the above, optical paths through both interferometers in the receiverare adjusted to track phase or frequency modulation by moving acomponent It will be understood that there are alternative arrangementsin which a path length change can be achieved by variation of refractiveindex rather than by physical movement and in some circumstances thismight be preferred. For instance, if one of the arms of eachinterferometer passes parallel to the other, it might be foundpreferable to insert a component which changes optical path length inboth arms simultaneously under electrical or thermal control, forinstance by changing the refractive index of an electro-optic orthermo-optic material of the component.

In an arrangement such as that shown in FIG. 7, it is possible to conveyboth voice and data information at the same time from the sametransmitter, for instance using phase or frequency modulation for voiceand using intensity modulation for data.

Wavelength Division Multiplexing Hybrid

Referring to FIG. 8, it is also possible to use embodiments of thepresent invention together with wavelength division multiplexing (WDM).

The specification of the International patent application publishedunder the number WO 0141346, entitled Multichannel Optical CommunicationSystem and Method Utilizing Wavelength and Coherence Multiplexing” dated2001-06-07 discloses a method and system for transmission of severalcoherence division multiplexed (CDM) optical signals via one WDMtransmission channel of a multichannel WDM telecommunication system. Abroadband optical source generates light within the spectral range of atleast one WDM transmission channel. Several CDM channels share thisspectral range to transmit and detect phase modulated optical signalsthrough optical fibre links.

It is however possible to use a spread spectrum signal 800 which extendsacross a spectrum much wider than a wavelength channel. The principle isillustrated in FIG. 8 which shows (in FIG. 8 a) the spectrum of WDMchannels 805 and the optical spread spectrum signal, together with (inFIG. 8 b) a possible receiver structure 810. FIG. 8 a shows the use ofan optical spectrum for CDM signals which is much wider than any of theindividual WDM channels. The receiver shown in FIG. 8 b will remove theWDM channels and then demodulate the remaining optical spectrum usingone or more interferometers. The fact that some optical power from thespread spectrum signal 800 has been extracted from the signal by theoptical filter which extracts the WDM channel 805 is not that importantbecause the information in the differential time delay system is encodedin the full spectrum of the optical spread spectrum.

Thus both WDM and time-delay difference (CDM) systems can be used overthe same optical channel yielding higher capacity communication andapproaching the theoretical limit of the optical communication channelcapacity.

Interferometric Sensing

In addition to the use of embodiments of the invention in voice and datacommunication channels, transmitters and receivers of the same generaltype can be used in telemetry for environmental parameters. For example,several interferometers with different respective optical pathimbalances can be connected to a network to sense environmentalparameters. The unique path imbalance of each interferometer serves toidentify it to a receiver. These parameters, such as pressure,temperature, vibration, magnetic field, electric field can be detectedif they cause a change in the time delay difference, phase and/or adifferential shift in the optical sensing interferometer and caneffectively be transmitted as data signals. The receiver used for suchtelemetry systems has the same structure as the one used forcommunication as described in this invention but will generally usedifferent signal processing techniques for gathering and interpretingthe data appropriately.

An embodiment of the invention thus might be described as an opticalnetwork as described in claim 1 of the accompanying claims, for use insensing one or more environmental parameters and transmitting aninformation-carrying optical communication signal wherein theinformation is representative of each said parameter, thetransmitting/receiving apparatus being adapted to sense said one or moreenvironmental parameters as interferometric modulation and to transmitsaid signal comprising said modulation.

1. An optical network for carrying communication signals to or from at least two end points in optical form, the network being adapted to provide voice communications and comprising: i) at least first and second end points, each comprising transmitting/receiving apparatus and having an address in the network; and ii) at least one optical source for delivering optical power to the transmitting/receiving apparatus of at least said first end point for conversion to electrical power at the transmitting/receiving apparatus and for transmitting an optical signal carrier to the transmitting/receiving apparatus of at least said first end point, wherein: (iii) the transmitting/receiving apparatus of the first end point comprises an interferometer for applying interferometric modulation comprising a voice signal and the address of the second end point to the optical signal carrier to provide an optical communication signal comprising the voice signal for subsequent transmission over the network to the second end point; at least one power conversion device for converting received optical power to electrical power; and at least one electrical storage device for storing at least a portion of the converted optical power; and (iv) the transmitting/receiving apparatus of the second end point comprises an interferometer for detecting interferometric modulation in said information-carrying optical communication signal received over the network so as to receive said voice signal.
 2. An optical network according to claim 1 wherein the transmitting/receiving apparatus of the first end point comprises a voice transducer for use in said interferometric modulation.
 3. An optical network according to claim 1 wherein, in use, said interferometric modulation comprises frequency modulation.
 4. Optical transmitting/receiving apparatus for use in said first end point of a network according to claim 1 to receive said optical power, wherein said apparatus also comprises an amplitude modulator to change the amplitude of said optical signal carrier so as to provide amplitude modulation.
 5. An optical network for carrying communication signals to or from at least two end points in optical form, the network comprising: i) at least first and second end points, each comprising transmitting/receiving apparatus; and ii) at least one optical source for delivering optical power to the transmitting/receiving apparatus of at least one of said end points for conversion to electrical power at the transmitting/receiving apparatus and for transmitting an optical signal carrier to the transmitting/receiving apparatus of at least said first end point, wherein the transmitting/receiving apparatus of the first end point comprises an interferometer for applying interferometric modulation to the optical signal carrier for subsequent transmission over the network as an information-carrying optical communication signal and the transmitting/receiving apparatus of the second end point comprises an interferometer for detecting interferometric modulation in said information-carrying optical communication signal received over the network; and wherein, in use, the optical signal carrier is used partly at the transmitting/receiving apparatus of at least one of the end points for conversion of delivered optical power to electrical power.
 6. An optical network for carrying communication signals to or from at least two end points in optical form, the network comprising: i) at least first and second end points, each comprising transmitting/receiving apparatus and having an address in the network; and ii) at least one optical source for delivering optical power to the transmitting/receiving apparatus of at least one of said end points for conversion to electrical power at the transmitting/receiving apparatus and for transmitting an optical signal carrier to the transmitting/receiving apparatus of at least said first end point, wherein the transmitting/receiving apparatus of the first end point comprises an interferometer for applying interferometric modulation comprising information and an address to the optical signal carrier to provide an information-carrying optical communication signal addressed to the address of the second end point for subsequent transmission over the network to said address and the transmitting/receiving apparatus of the second end point comprises an interferometer for detecting the interferometric modulation in said addressed, information-carrying optical communication signal received over the network, so as to receive the information; and wherein, in use, said interferometric modulation comprises one or more interference fringes created by passing an optical carrier through the interferometer of the first end point.
 7. An optical network for carrying communication signals to or from at least two end points in optical form, for use in sensing one or more environmental parameters and transmitting said information-carrying optical communication signal wherein the information is representative of each said parameter, the network comprising: i) at least first and second end points, each comprising transmitting/receiving apparatus; and ii) at least one optical source for delivering optical power to the transmitting/receiving apparatus of at least one of said end points for conversion to electrical power at the transmitting/receiving apparatus and for transmitting an optical signal carrier to the transmitting/receiving apparatus of at least said first end point, wherein the transmitting/receiving apparatus of the first end point comprises an interferometer for applying interferometric modulation to the optical signal carrier for subsequent transmission over the network as an information-carrying optical communication signal and the transmitting/receiving apparatus of the second end point comprises an interferometer for detecting interferometric modulation in said information-carrying optical communication signal received over the network, and wherein the transmitting/receiving apparatus of the first end point is adapted to sense said one or more environmental parameters as interferometric modulation and to transmit said information-carrying optical communication signal comprising said modulation.
 8. Optical transmitting/receiving apparatus for use in an end point of an optical network, the transmitting/receiving apparatus being adapted for receiving optical power and for transmitting an information-carrying optical communication signal comprising a voice signal, the network being adapted for carrying said communication signal, the network comprising: i) at least first and second end points, each comprising transmitting/receiving apparatus and having an address in the network; and ii) at least one optical source for delivering optical power to the transmitting/receiving apparatus of at least said first end point for conversion to electrical power at the transmitting/receiving apparatus and for transmitting an optical signal carrier to said transmitting/receiving apparatus, wherein the transmitting/receiving apparatus for use in said first end point comprises: a) an interferometer for applying interferometric modulation comprising a voice signal and the address of the second end point to the optical signal carrier to provide the information-carrying optical communication signal comprising a voice signal, for subsequent transmission over the network to the transmitting/receiving apparatus of the second end point, which apparatus comprises an interferometer for detecting interferometric modulation in said information-carrying optical communication signal so as to receive said voice signal, b) at least one power conversion device for converting received optical power to electrical power, and c) at least one electrical storage device for storing at least a portion of the converted optical power.
 9. Optical transmitting/receiving apparatus according to claim 8, further comprising an additional optical source.
 10. Optical transmitting/receiving apparatus according to claim 8, said apparatus being provided with a local electrical power source.
 11. Optical transmitting/receiving apparatus according to claim 8, for use in said first end point, wherein the optical paths in the interferometer are mismatched, in use, by an optical path difference which is greater than the coherence length of the optical signal carrier carrying the modulation.
 12. Optical transmitting/receiving apparatus for use in an end point of an optical network, the transmitting/receiving apparatus being adapted to receive optical power and to transmit an information-carrying optical communication signal comprising a voice signal, the network being adapted for carrying said communication signal, the network comprising: i) at least first and second end points, each comprising transmitting/receiving apparatus and having an address in the network; and ii) at least one optical source for delivering optical power to the transmitting/receiving apparatus of at least said first end point for conversion to electrical power at the transmitting/receiving apparatus and for transmitting an optical signal carrier to said transmitting/receiving apparatus, wherein the transmitting/receiving apparatus for use in said first end point comprises: a) an interferometer for applying interferometric modulation to the optical signal carrier to provide the information-carrying optical communication signal addressed to the address of the second end point, for subsequent transmission over the network to the transmitting/receiving apparatus of the second end point, which apparatus comprises an interferometer for detecting interferometric modulation in said information-carrying optical communication signal so as to receive said voice signal, and b) at least one power conversion device for converting received optical power to electrical power and applying the electrical power to drive one or more components of the transmitting/receiving apparatus.
 13. Optical transmitting/receiving apparatus according to claim 12, for use in said first end point, wherein the optical paths in the interferometer are mismatched, in use, by an optical path difference which is greater than the coherence length of the optical signal carrier carrying the modulation.
 14. Optical transmitting/receiving apparatus for use in an end point of an optical network to receive optical power and an optical signal carrier, the network comprising: i) at least first and second end points, each comprising transmitting/receiving apparatus; and ii) at least one optical source for delivering optical power to the transmitting/receiving apparatus of at least said first end point for conversion to electrical power at the transmitting/receiving apparatus and for transmitting an optical signal carrier to said transmitting/receiving apparatus, wherein the transmitting/receiving apparatus for use in said first end point comprises an interferometer for applying interferometric modulation to the optical signal carrier for subsequent transmission over the network as an information-carrying optical communication signal to the transmitting/receiving apparatus of the second end point, which apparatus of the second end point comprises an interferometer for detecting interferometric modulation in said information-carrying optical communication signal, and wherein the interferometer of the transmitting/receiving apparatus for use in said first end point comprises means to change the length of at least one arm of the interferometer to create a frequency shift in said optical signal carrier so as to provide said information-carrying optical communication signal.
 15. Optical receiving apparatus for use in an end point of an optical network, the receiving apparatus being adapted to receive optical power and an information-carrying optical communication signal, the network being adapted for carrying said communication signal, the network comprising: i) at least first and second end points, the first end point comprising transmitting/receiving apparatus and the second end point comprising the optical receiving apparatus; and ii) at least one optical source for delivering optical power to the transmitting/receiving apparatus of at least one end point for conversion to electrical power at the transmitting/receiving apparatus and for transmitting an optical signal carrier to said transmitting/receiving apparatus of at least said first end point, wherein the transmitting/receiving apparatus of said first end point comprises an interferometer for applying interferometric modulation to the optical signal carrier for subsequent transmission over the network as the information-carrying optical communication signal to the optical receiving apparatus, and wherein said optical receiving apparatus is provided with at least two interferometers for detecting interferometric modulation in a received information-carrying optical communication signal, a first interferometer being arranged for use in detecting a first type of modulation in the received signal and a second interferometer being arranged for use in detecting a second type of modulation in the received signal.
 16. Optical receiving apparatus according to claim 15 wherein the first type of modulation comprises phase or frequency modulation.
 17. Optical receiving apparatus according to claim 15 wherein the second type of modulation comprises intensity modulation.
 18. Optical receiving apparatus according to claim 15 wherein the at least two interferometers are differently unbalanced, the discrepancy between the path length differences of the interferometers being equal to, or approximately equal to, a quarter of a central wavelength of an optical carrier carrying the received signal.
 19. Optical receiving apparatus according to claim 15 further comprising a path length control component for producing a path length change in relation to both interferometers of the at least two interferometers, at the same time.
 20. Optical receiving apparatus according to claim 19 wherein the at least two interferometers share at least one reflector as the path length control component, the arrangement being such that movement of the shared reflector results in said path length change in relation to both interferometers.
 21. Optical receiving apparatus according to claim 15 wherein the at least two interferometers are both provided by one shared pair of reflectors, at least one of which is a retroreflector, the apparatus further comprising at least one input beam splitter, the arrangement being such as to provide multiple optical paths in relation to the reflectors which can be brought together as outputs to constitute said at least two interferometers.
 22. Optical receiving apparatus according to claim 19, comprising tracking means for tracking phase or frequency modulation in the received signal and a feedback arrangement for controlling the path length control component such that both interferometers are adjusted to follow said phase or frequency modulation.
 23. Optical transmitting apparatus for use with apparatus according to claim 15, the transmitting apparatus comprising an intensity modulator for applying intensity modulation to said optical signal carrier so as to provide said information-carrying optical communication signal.
 24. An optical receiver adapted to receive optical power and an information-carrying optical communication signal, for use in transmitting/receiving apparatus at an end point of an optical network, the network being adapted for carrying said communication signal and comprising: i) at least first and second end points each having an address in the network, the first end point comprising transmitting/receiving apparatus and the second end point comprising the optical receiver; and ii) at least one optical source for delivering optical power to the transmitting/receiving apparatus of at least one end point for conversion to electrical power at the transmitting/receiving apparatus and for transmitting an optical signal carrier to said transmitting/receiving apparatus of at least said first end point, wherein the transmitting/receiving apparatus of said first end point comprises an interferometer for applying interferometric modulation to the optical signal carrier to provide the information-carrying optical communication signal addressed to the address of the second end point, for subsequent transmission over the network to the optical receiver, said receiver comprising a filter for filtering out one or more wavelength ranges from said communication signal, and wherein the receiver further comprises an interferometer for use in detecting, in a portion of said communication signal outside said one or more wavelength ranges, interferometric modulation generated using a transmitting interferometer with a characteristic path length difference so as to provide the address of the second end point.
 25. Optical transmitting/receiving apparatus for use in a first end point of an optical network, the transmitting/receiving apparatus being adapted for receiving optical power and for transmitting an information-carrying optical communication signal, the network being adapted for carrying said communication signal and comprising: i) at least said first end point and a second end point, each comprising transmitting/receiving apparatus; and ii) at least one optical source for delivering optical power to the transmitting/receiving apparatus of at least said first end point for conversion to electrical power at the transmitting/receiving apparatus and for transmitting an optical signal carrier to said transmitting/receiving apparatus, wherein the transmitting/receiving apparatus for use in said first end point comprises: a) an interferometer for applying interferometric modulation to the optical signal carrier for subsequent transmission over the network as the information-carrying optical communication signal, b) at least one power conversion device for converting received optical power to electrical power, c) at least one electrical storage device for storing at least a portion of the converted optical power, and d) a further interferometer for detecting interferometric modulation in an information-carrying optical communication signal received over the network.
 26. Optical transmitting/receiving apparatus for use in a first end point of an optical network, the transmitting/receiving apparatus being adapted to receive optical power and to transmit an information-carrying optical communication signal, the network being adapted for carrying said communication signal and comprising: i) at least said first end point and a second end point, each comprising transmitting/receiving apparatus; and ii) at least one optical source for delivering optical power to the transmitting/receiving apparatus of at least said first end point for conversion to electrical power at the transmitting/receiving apparatus and for transmitting an optical signal carrier to said transmitting/receiving apparatus, wherein the transmitting/receiving apparatus for use in said first end point comprises: a) an interferometer for applying interferometric modulation to the optical signal carrier for subsequent transmission over the network as the information-carrying optical communication signal, b) at least one power conversion device for converting received optical power to electrical power and applying the electrical power to drive one or more components of the transmitting/receiving apparatus, and c) a further interferometer for detecting interferometric modulation in an information-carrying optical communication signal received over the network.
 27. Optical receiving apparatus for receiving optical power and optical signals delivered to an end point over an optical network which carries communication signals to or from the one end point in optical form, the optical network comprising: i) the end point comprising transmitting/receiving apparatus; ii) at least one optical source for delivering optical power to the transmitting/receiving apparatus for conversion to electrical power at the transmitting/receiving apparatus and for transmitting an optical signal carrier to the transmitting/receiving apparatus, wherein the transmitting/receiving apparatus is adapted to apply interferometric modulation to the optical signal carrier for subsequent transmission over the network as an information-carrying optical communication signal; wherein the optical receiving apparatus is provided with at least two interferometers for detecting interferometric modulation in a received signal, a first interferometer being arranged for use in detecting a first type of modulation in the received signal and a second interferometer being arranged for use in detecting a second type of modulation in the received signal; wherein the optical receiving apparatus further comprises a path length control component for producing a path length change in relation to both interferometers at the same time; and wherein the at least two interferometers share at least one reflector as the path length control component, the arrangement being such that movement of the shared reflector results in said path length change in relation to both interferometers.
 28. Optical receiving apparatus for receiving optical power and optical signals delivered to an end point over an optical network which carries communication signals to or from the one end point in optical form, the optical network comprising: i) the end point comprising transmitting/receiving apparatus; ii) at least one optical source for delivering optical power to the transmitting/receiving apparatus for conversion to electrical power at the transmitting/receiving apparatus and for transmitting an optical signal carrier to the transmitting/receiving apparatus, wherein the transmitting/receiving apparatus is adapted to apply interferometric modulation to the optical signal carrier for subsequent transmission over the network as an information-carrying optical communication signal; wherein the optical receiving apparatus is provided with at least two interferometers for detecting interferometric modulation in a received signal, a first interferometer being arranged for use in detecting a first type of modulation in the received signal and a second interferometer being arranged for use in detecting a second type of modulation in the received signal; and wherein the at least two interferometers are both provided by one shared pair of reflectors, at least one of which is a retroreflector, the apparatus further comprising at least one input beam splitter, the arrangement being such as to provide multiple optical paths in relation to the reflectors which can be brought together as outputs to constitute said at least two interferometers.
 29. Optical receiving apparatus for receiving optical power and optical signals delivered to an end point over an optical network which carries communication signals to or from the one end point in optical form, the optical network comprising: i) the end point comprising transmitting/receiving apparatus; ii) at least one optical source for delivering optical power to the transmitting/receiving apparatus for conversion to electrical power at the transmitting/receiving apparatus and for transmitting an optical signal carrier to the transmitting/receiving apparatus, wherein the transmitting/receiving apparatus is adapted to apply interferometric modulation to the optical signal carrier for subsequent transmission over the network as an information-carrying optical communication signal; wherein the optical receiving apparatus is provided with at least two interferometers for detecting interferometric modulation in a received signal, a first interferometer being arranged for use in detecting a first type of modulation in the received signal and a second interferometer being arranged for use in detecting a second type of modulation in the received signal; wherein the optical receiving apparatus further comprises a path length control component for producing a path length change in relation to both interferometers at the same time; and wherein the optical receiving apparatus further comprises tracking means for tracking phase or frequency modulation in the received signal and a feedback arrangement for controlling the path length control component such that both interferometers are adjusted to follow said phase or frequency modulation. 